For two stage culture with control of tryptophan concentration, rapid ill situ removal of tryptophan by adding adsorbent was investigated. Activated carbon, BAC, could adsorb a larger amount of tryptophan than other adsorbents. From test tube cultures, it was found that 20 g/l of BAC was necessary to remove large portions of tryptophan in a medium containing 20 mg/l of tryptophan and induce the beta-galactosidase gene expression. Two stage culture was performed using a jar fermentor in which tryptophan concentration was kept at 20 mg/l in the first stage for cell growth and then almost totally removed by direct addition of 20 g/l of BAC to the medium. One hour after addition of BAC, an increase in enzyme activity was observed. Maximum specific enzyme activity reached a final value of 320 mu kat/g-protein, which corresponded to a level 12 times that of the control culture.